Silicon carbide (SiC) has a dielectric breakdown electric field that is an order of magnitude larger than that of silicon (Si), and a band gap that is three times as wide. Further, silicon carbide (SiC) has thermal conductivity that is about three times higher than that of silicon (Si). Silicon carbide has these types of characteristic properties. As a result, silicon carbide (SiC) holds much promise for applications to power devices, high-frequency devices, and high-temperature operation devices and the like.
In recent years, in order to obtain many semiconductor devices from a single substrate, increases in the diameter of SiC single crystal substrates have been sought. Accordingly, demand for SiC single crystals of larger diameter is also growing.
SiC single crystal substrates are produced by cutting a SiC ingot. These SiC ingots are generally obtained by the sublimation method. The sublimation method is a method in which a seed crystal composed of a SiC single crystal is placed on a pedestal located inside a graphite crucible, and by heating the crucible, a sublimed gas produced by sublimation of a raw material powder provided inside the crucible is supplied to the seed crystal, thereby growing the seed crystal into a larger SiC ingot.
However, obtaining a large SiC ingot directly from a small seed crystal is difficult. As a result, the seed crystal is generally grown into a large SiC single crystal, with that SiC single crystal then being used to produce the SiC ingot.
It is common knowledge in the field of SiC single crystals that when growing a large SiC single crystal from a small seed crystal, the large SiC single crystal cannot be obtained in a single growth step. Accordingly, crystal growth from the seed crystal is performed over a plurality of crystal growth repetitions to obtain a large SiC single crystal.
During each crystal growth repetition, the crystal growth direction of the growing crystal is important. FIG. 1 is a schematic illustration for describing crystal orientations and crystal planes. In SiC single crystals, the main known crystal planes are the {0001} plane (c-plane), and the 1-1001 plane (m plane) and {11-20} plane (a-plane) which are perpendicular to the c-plane. The “-” symbol in the crystal plane indices is typically included above the corresponding number, but in this description and in the drawings, for the sake of convenience, the symbol is included to the left of the number. Further, in the case of <0001>, <1-100> and <11-20> which indicate crystal orientations, the symbol “-” is used in the same manner. In the following description, crystal growth in the <0001> direction is also referred to as “c-plane growth”, crystal growth in the <1-100> direction is also referred to as “m-plane growth”, and crystal growth in the <11-20> direction is also referred to as “a-plane growth”.
The brackets { } which show the indices that indicate the plane, and the brackets < > which show the indices that indicate direction show planes and directions having equivalent symmetry, and therefore do not differentiate orientation. In the present description, when differentiating planes or directions, differentiation is sometimes made on the basis of “orientation”. For example, when one surface of a crystal having {11-20} planes on both the front and back surfaces is deemed the front surface, the back surface may sometimes be referred to as “the surface of opposite orientation”.
When a crystal is grown in the c-plane direction from a seed crystal, a known problem occurs in that the single crystal obtained following growth contains an extremely large number of defects such as micropipe defects and threading screw dislocations in directions parallel to the <0001> direction.
As a result, one known method that is used for obtaining a large SiC single crystal from a small seed crystal is the RAF (Repeated a-face) method disclosed in Patent Documents 1 and 2. The RAF method is a method in which a-plane growth is performed at least once, and then c-plane growth is performed. By using the RAF method, a SiC single crystal having almost no screw dislocations or stacking faults can be produced. This is because any screw dislocations or stacking faults within the SiC single crystal following the a-plane growth are not inherited in the SiC single crystal obtained following c-plane growth. A SiC single crystal obtained following a-plane growth has stacking faults formed parallel to the <11-20> direction. However, in c-plane growth, crystal growth occurs in a direction perpendicular to the direction in which these stacking faults have been formed. Consequently, a SiC single crystal obtained by performing c-plane growth after a-plane growth has almost no screw dislocations or stacking faults.
Furthermore, Patent Document 3 discloses a method for producing a SiC ingot in which, in order to further suppress screw dislocations and stacking faults, a grown SiC single crystal is cut at a prescribed angle, and the SiC ingot is then produced based on this cut SiC single crystal.